520 



CELL MECHANICS 



The third force is an attraction wliich is specific to the parts of the 

 chromosomes and is shown only by their ends. We now have to 

 consider how these forces, inferred from the relatively simple 

 system of the prophase nucleus, act when the chromosomes are 

 brought into a specially differentiated substrate, during the extra- 

 nuclear stages of their life-cycle. 



(iv) Balanced Repulsions in the Spindle, (a) Introduction. The 

 metaphase and anaphase movements of the chromosomes are more 

 complicated than the prophase movements, for an obvious reason. 

 During the prophase the chromosomes are free from outside inter- 

 ference. Their movements show what they can do by themselves. 



Fig. 148. — Diagram to show the effect of dehydration on the different 

 cell constituents at metaphase of mitosis or meiosis. Chromo- 

 somes black, cytoplasm stippled, spindle clear, a In isotomic 

 solution, b, c, d In hypertonic solution, successive stages of 

 dehydration, a-c axial section ; d equatorial section. (From 

 Belar, 1927.) 



At metaphase they are brought into relation with another set of 

 variable conditions, conditions bound up with the activity of the 

 spindle. It is only therefore, with a clear understanding of the 

 simpler movements that take place in the prophase nucleus that 

 we can hope to consider their later activities profitably. This 

 we are now in a position to do. 



(h) The Structure of the Spindle. The special properties of the 

 spindle as distinct from the cytoplasm may be inferred in three 

 ways (Belar, 1929, a and h). When the cytoplasm is shrunk by 

 dehydration of living cells in a hypertonic solution, as in the course 

 of fixation or in special experiments, the spindle shrinks less than 

 the rest. When living cells are smeared on a slide in a single layer, 

 as is readily done when the cells are large, they lie with their 



